According to dbaron's instruction in bug 698783 comment 12, I believe all the prerequisites to remove moz-prefixed multi-column properties (including column-gap defined in CSS alignment) have been done.

We've shipped all the multi-column properties (including column-span). The internal usages of the properties have been converted to standard ones in bug 1308587, although there are still some missed that I'll address in this bug.

This patch was generated by the following shell script.

function remove_column_prefix() {
    echo "Renaming $1 to $2"
    find .\
         -type f\
         ! -path "./obj*"\
         ! -path "./.git"\
         ! -path "./.hg"\
         \( -name "*.html" -or\
            -name "*.xhtml" -or\
            -name "*.xht" -or\
            -name "*.xul" -or\
            -name "*.xml" -or\
            -name "*.svg"  \)\
            -exec sed -i -e "s/$1/$2/g" "{}" \;
}
remove_column_prefix "MozColumn" "column"

Depends on D59562

The modification to properties-db.js was generated by
./mach devtools-css-db.

Depends on D59563

-moz-column-span missed the conversion in Bug 1308587 Part 2.
https://hg.mozilla.org/mozilla-central/rev/bb59a0c37162

I'm a bit uneasy doing this. Like sure, it's easy, but aliases are cheap and there's background for some sites (mainly google properties) only sending us -moz- prefixed stuff.

Is there a benefit of removing the aliases from the engine?

I agree with emilio here. Can we at least stick it behind a pref that ships to pre-release for a few releases to get a sense of any potential fallout?

Is there a benefit of removing the aliases from the engine?

Since it is easy to remove the prefix, the implementation isn't simplified as much. However, I feel this is a question of doing now or never.

I agree with emilio here. Can we at least stick it behind a pref that ships to pre-release for a few releases to get a sense of any potential fallout?

I agree it's better to unprefix in pre-release for a few releases for any web-compat fallout. dbaron suggests this in the review as well
https://phabricator.services.mozilla.com/D59564#1816396

I think it's easy to switch the prefix behind a pref like
https://searchfox.org/mozilla-central/rev/d4d6f81e0ab479cde192453bae83d5e3edfb39d6/servo/components/style/properties/longhands/position.mako.rs#318

In the latest revision, I added a pref layout.css.prefixes.columns to guard the prefixed properties behind, and use false by default for all channels.

Posted site compatibility note.

https://hg.mozilla.org/mozilla-central/rev/21f9476a9588
https://hg.mozilla.org/mozilla-central/rev/08e88920caa4
https://hg.mozilla.org/mozilla-central/rev/b7913d967261

Documentation complete, see https://github.com/mdn/sprints/issues/2739#issuecomment-583468459